I got my account downgraded a month ago and received all the required emails. Moreover, I don't even have a billing tab to access my billing history and your history tracker indicates that I have no history whatsoever. Despite all of this, I got charged for a new billing cycle. Not only you don't have an adequate system of communication with your help line, nor your chat bot is helpful as it constantly directs me to your cancellation page. It seems quite unprofessional and unhelpful that I have to ask a community to attract your attention!! Please attend to my matter AS A MATTER OF URGENCY.
If you're not seeing any payment, even old ones, and yet you're being charged, then you must have another account on Dropbox which is being charged that you're not aware of. Are you certain you're signed into the correct email address?
On your bank statement, does it show Dropbox, iTunes or Google Play? If it shows Dropbox, try using the card lookup tool to find out which account it belongs to.